Theoretical insight on the origin of anelasticity in zinc oxide nanowires

Room-temperature oxygen vacancy migration induced reversible phase transformation during the anelastic deformation in CuO

From the mechanical perspectives, the influence of point defects is generally considered at high temperature, especially when the creep deformation dominates. Here, we show the stress-induced reversible oxygen vacancy migration in CuO nanowires at room temperature, causing the unanticipated anelastic deformation. The anelastic strain is associated with the nucleation of oxygen-deficient CuO_x phase, which gradually transforms back to CuO after stress releasing, leading to the gradual recovery of the nanowire shape. Detailed analysis reveals an oxygen deficient metastable CuO_x phase that has been overlooked in the literatures. Both theoretical and experimental investigations faithfully predict the oxygen vacancy diffusion pathways in CuO. Our finding facilitates a better understanding of the complicated mechanical behaviors in materials, which could also be relevant across multiple scientific disciplines, such as high-temperature superconductivity and solid-state chemistry in Cu-O compounds, etc.

The effect of point defects on mechanical behaviour of materials is generally considered at high temperatures. This work reports a reversible stress-induced migration of point defects during anelastic deformation in CuO nanowires at room temperature resulting from heterogeneous strain distribution.

Phase transformation and its effect on the piezopotential in a bent zinc oxide nanowire

Most piezotronic nanodevices rely on the piezopotential generated by the bending of their component piezoelectric nanowires (NWs). The mechanical behaviours and piezopotential properties of zinc oxide (ZnO) NWs under lateral bending are investigated in this paper by using a multiscale modelling technique combining first-principles calculations, molecular dynamics simulations and finite-element calculations. Two phase transformation processes are successively found in ZnO NWs by increasing the bending force. As a result, the inner and outer surfaces of bent ZnO NWs transform from the parent wurtzite (WZ) structure to a hexagonal (HX) structure and a body-centred-tetragonal (BCT-4) structure, respectively. Different material properties are found among the WZ, BCT-4, and HX structures, which result in a significant change in the piezopotential distribution in bent ZnO NWs after the phase transformation. Meanwhile, the piezopotential generated in bent ZnO NWs can be enhanced by an order of magnitude due to the phase transformation. Moreover, a significant increase in the electronic band gap is found in the transformed HX structure, which implies that the phase transformation may also affect the piezopotential in bent ZnO NWs by modifying their semiconducting properties especially when the doping level of NWs is large.